This invention relates generally to optical communication networks and in particular to a remote optical bypass switch for use in such a network.
Fiber optic local area networks (LAN's) using a ring topology require a bypass switch at each node where an optical signal carried on the ring is received and retransmitted. The bypass switch allows the transmission of information on the network if there is a failure in the regeneration of the signal at the associated node, whether this failure is due to a malfunction of equipment at the node, such as the failure of a transmitter LED, or a power failure. The optical signal is transmitted along the ring, through the bypass switch, without interfacing with the terminal or other equipment at the node. The node is therefore "bypassed" and optical energy carrying information continues to circulate through the network. In usual commercial fiber optic LAN's, bypass switches are integrated into each user terminal.
In military applications where buildings containing terminals may be prime targets for attack, it is desirable to place the bypass switch at a location remote from the terminals and preferably in a hardened environment. The remote location increases the survivability of the network. The present solution to providing a remote bypass capability has been to connect the terminal to the remote bypass switch with a hybrid stub cable, one containing optical cables to interface with the transmission system and copper cables that carry an electrical current sufficient to hold an optical relay in a non-bypassed mode. A control signal, usually developed at the terminal, is transmitted over the copper cables to the remote bypass switch. If power is lost, the relay operates to place the switch in a bypass mode. The relay can also operate, as by interrupting the holding current, if a failure is detected at the terminal equipment.
A drawback is that the copper cables leave the system susceptible to electromagnetic interference (EMI), whether man-made or natural, e.g. lightning. Another drawback in known fiber optic LAN's is that it is possible to lose an optical input at a defective node because the bypass switch is responsive only to a power failure or interruption. For example, a break in an optical fiber in the stub cable or a transmitter LED at the terminal which has a diminished output, but is still functioning to some degree can both result in the loss of information without triggering the operation of a conventional remote optical bypass switch.
It is therefore a principal object of the present invention to provide a remote optical bypass switch for a fiber optic LAN with a ring topology which detects and reliably operates in response to a power failure as well as a malfunction or destruction of an associated terminal.
Another object is to provide a remote bypass switch with the foregoing objects which can be automatically reset.
A further object is to provide a remote bypass switch which can operate in association with a purely dielectric stub cable and which therefore is significantly less susceptible to EMI than present commercial remote bypass switches.
Another object is to provide a remote bypass switch which can operate with all of the foregoing advantages during a power failure.
Still another object of the present invention is to provide a remote bypass switch with the foregoing advantages that can be manufactured using conventional components.